1. Field of the Invention
The present invention relates to an improvement in a fluorescent composition for low velocity electron beam excitation, which exhibits emission of light of high luminance under excitation with a low velocity electron beam at an acceleration voltage of not higher than 1 kV, (hereinafter referred to simply as a "fluorescent composition").
2. Discussion of Background
Phosphors having high electrical resistance including a series of sulfide phosphors consisting essentially of a sulfide of the formula (Zn.sub.1-x,Cd.sub.x)S, wherein x is a number satisfying a condition Of 0.ltoreq.x.ltoreq.1, as the host material, which is activated by zinc (Zn), silver (Ag), gold (Au), copper (Cu), manganese (Mn) or the like and further coactivated by halogen, aluminum (Al), gallium (Ga), indium (In) or the like, as well as oxyacid phosphors such as cerium (Ce)-activated yttrium aluminate.multidot.gallate phosphor {Y.sub.3 (Al,Ga).sub.5 O.sub.12 :Ce} and europium (Eu)-activated barium.magnesium alminate phosphor {(Ba,Mg)0.6Al.sub.2 O.sub.3 :Eu}, and oxide phosphors such as europium (Eu)-activated yttrium oxide phosphor (Y.sub.2 O.sub.3 :Eu), do not substantially emit light under excitation by a low velocity electron beam, although they emit light well under excitation by a high velocity electron beam at an acceleration voltage of higher than 1 kV. Fluorescent compositions obtained by mixing conductive materials such as indium oxide (In.sub.2 O.sub.3), zinc oxide (ZnO) and tin oxide (SnO.sub.2) with such phosphors having high electrical resistance, exhibit visible emissions of high luminance ranging from blue to red specific to the phosphors which are the main components of the fluorescent compositions, under excitation by a low electron beam at an acceleration voltage of not higher than 1 kV (Japanese Examined Patent Publications No. 33153/1984, No.33155/1984, No. 44035/1987 and No. 53554/1987). Therefore, such fluorescent compositions have recently found much use as the fluorescent screens for fluorescent display tubes capable of multi-color displaying in combination with ZnO:Zn which has been used since old as a phosphor showing greenish white emission, which are useful for automobiles (as displays for clocks, car audios, speed meters or tachometers), audio-visual appliances, video tape recorders (VTR), etc.
However, especially in the case of fluorescent display tubes for automobiles, there is a limitation in the acceleration voltage for the fluorescent display tubes in view of the power sources to be used, whereby according to the conventional technique, the luminance of the fluorescent display tubes is inadequate, particularly it lacks in the visibility under the external daylight, and this causes a problem from the viewpoint of the safe driving of the automobiles.